Abstract : Wavelength discrimination functions were measured with square-wave grating test objects in which alternate sets of bars were illuminated with two different wavelengths. When both wavelengths are made equal in brightness, the wavelength difference required to detect the grating increases monotonically with grating spatial frequency. Since this relation is approximately constant across the visible spectrum, the shape of the wavelength discrimination function tends to be preserved at high spatial frequencies, although much higher wavelength differences are needed to detect the high frequency gratings. Introducing a detectable brightness mismatch between the two wavelengths results in (1) a reduction of the wavelength difference needed to detect a hue difference between adjacent grating bars and (2) a minimum in the threshold wavelength difference versus spatial frequency function. Under these conditions, wavelength difference no longer increases monotonically with spatial frequency. The resulting functions resemble those previously reported for threshold luminance contrast versus spatial frequency. The relation of these results to form detection and to lateral inhibition is discussed. (Author)